The present invention relates to an image display apparatus and, more particularly, to a portable head- or face-mounted image display apparatus which can be retained on the user's head or face.
In recent years, helmet- and goggle-type head-mounted image display apparatuses, which are designed to be retained on the user's head or face, have been developed for the purpose of enabling the user to enjoy virtual reality or a wide-screen image by oneself.
For example, Japanese Patent Application Laid-Open (KOKAI) No. 03-191389 (1991) discloses an image display apparatus which has, as shown in FIG. 12, a two-dimensional image display device 1 for displaying the content of information, a concave mirror 2 provided to face the display device 1 to project the display content on a user's eyeball as an enlarged image, and a semitransparent mirror 3 disposed between the display device 1 and the concave mirror 2, thereby enabling a wide-screen image to be obtained with a compact structure. If the semitransparent mirror 3 is adapted to pass also an outside world image, the user can observe the outside world image superimposed on the image displayed on the display device 1. To increase the working distance between the user's eyeball and the ocular optical system, the semitransparent mirror 3 may be replaced with a beam splitter prism 4 having a semitransparent surface, as shown in FIG. 13. In such a case, a reverse reflecting mirror 5 is used in place of the concave mirror 2. These ocular optical systems are designed on the assumption that the angle of view is about 30.degree. in the horizontal direction, that is, in the direction of the x-z plane. The view angle of 30.degree. is equivalent to a view angle where one watches a 26-inch television at a distance of 1 m. FIG. 14 graphically shows spherical aberration, astigmatism, distortion and lateral aberration at the horizontal view angle of 30.degree. in the ocular optical system arranged as shown in FIG. 13.
In order to further enhance the feeling of being at the actual spot, which is given to the user when viewing the displayed image, the horizontal view angle of the image that is projected on the user's eyeball as an enlarged image must be made larger than 30.degree.. The above-described conventional ocular optical system is compact, and yet it is capable of effectively correcting various aberrations, as shown in FIG. 14; this is a very important feature for a head-mounted image display apparatus. Accordingly, it is important to make the horizontal view angle greater than 30.degree. while effectively using the advantageous feature. It is obvious in an actual ocular optical system having the above-described arrangement that the size of the prism increases as the angle of view is increased. The increase in the size of the prism causes the ocular optical system to increase in both size and weight; this is a fatal problem. The problem may be solved by providing a positive lens between the user's eyeball and the prism to bend the rays of light so that the prism can be made as compact as possible. However, in such an optical system, the most off-axis distortion becomes 10% or more, so that the distortion of the image at the edge of image field becomes large. Thus, the image provided by the optical system fails to allow the user to enjoy the feeling of being at the actual spot. The results of actual observation made by the present applicant have revealed that as long as the distortion is not larger than about 5%, the user can enjoy watching the displayed image without impairing the feeling of being at the actual spot. Thus, it is essential to reduce the distortion to about 5% by aberration correction even in a case where the angle of view is 30.degree. or more.